In recent years, for the purpose of reducing environmental loads, strict regulations against exhaust gases have been successively introduced in the automobile industry. The exhaust gases contain, in addition to carbon dioxide (CO2) as a global worming substance, various harmful substances such as particulate matters (PM), hydrocarbons (HC), carbon monoxide (CO) and nitrogen oxides (NOx). Among these substances, very strict regulation values have been imposed on PM and NOx. As the measure for reducing an amount of these substances discharged, gasoline automobiles are provided with a three-way catalyst, whereas diesel automobiles are provided with a diesel particulate filter (DPF). The exhaust gases are cleaned by passing through these members, and then discharged into atmospheric air.
Meanwhile, it has recently reported that active sites of the three-way catalyst tend to be poisoned with phosphorus components in engine oils to thereby cause deterioration in a catalyst performance thereof, and that ash derived from metal components is deposited on the DPF to thereby reduce the service life of the DPF. At present, in the ILSAC Standard and the JASO Standard as standards for engine oils, the upper limits of the phosphorus content and ash content in engine oils have been established, and the engine oils having lower contents of these substances have now been developed.
In recent years, from the viewpoint of improving fuel consumption, parts of an engine or a transmission are formed of a nonferrous metal material for reducing the weight thereof. Among nonferrous metal materials, an aluminum alloy, in particular, an Al—Si alloy, has been frequently employed. However, conventional engine oils contain an anti-wear agent such as zinc dithiophosphate (ZnDTP), which is intended to mainly induce reaction for forming a coating film on Fe. Therefore, there is concern about deterioration of wear resistance of such an oil with respect to aluminum materials such as an Al—Si alloy.
Therefore, studies have been conducted to provide an anti-wear agent suitable for aluminum materials, as disclosed in, for example, Patent Document 1.